Attention is directed to commonly assigned, copending patent application, U.S. Ser. No. 319,177 by M. S. Wrighton and D. C. Bookbinder entitled "Synthesis of N,N'-Dialkyl-4,4'Bipyridinium Reagents", filed July 14, 1981, disclosing a novel class of viologens (4,4'bipyridinium compounds) which may be polymerized and covalently bonded to surfaces as well as the use of these viologens in electrochromic (color-changing) display devices; this application is hereby incorporated by reference. This copending application identifies a number of binding groups, such as silicon alkoxides, silicon halides, boron alkoxides and boron halides, which are useful in binding viologens to substrates.
Generally, in redox catalysis, one is concerned with lowering the kinetic barrier to electron transfer from one molecule (or a surface) to another molecule. One area of interest is electron-reduction of materials, particularly biological materials, at surfaces, which surfaces may be conducting, semiconducting or insulating depending on the application. Another area of interest concerns hydrogen evolution (i.e. in electrolysis) and the reverse reaction, hydrogen oxidation (i.e. electricity generating fuel cells).
Reductions of complex biological materials pose special problems. Most redox agents are highly selective in their actions on various molecules. Care must also be taken to avoid unacceptable by-products. For example, when a redox agent reduces biological molecule, it yields an oxidized by-product which is often unacceptable to the biological system of interest. These by-products typically must be removed by tedious chromatography or dialysis.
One alternative is electrochemical reduction using an electrode at the proper potential to induce the desire reaction. However, when the necessary reduction involves metal-containing macromolecules having an electron-transfer function, the molecules often do not respond at the conventional electrode because the molecule's own redox center (typically a chelated metal ion) can not come close enough to the electrode.
Moreover, electrochemical reactions may be stymied by fouling of the electrode surface as a result of non-specific adsorption of the biological material onto the electrode. Thus, there exists a need for novel redox agents which can induce reactions without adverse by-products and which can be used in reducing complex biological materials.
There also exists a need for redox mediating agents which can improve electrolysis reactions, particularly photoelectrochemical hydrogen generation. In order to decompose water in an electrolytic cell a photocathode must present excited electrons as reducing equivalents with enery sufficient to reduce H.sup.+ in solution; at this point H.sub.2 is thermodynamically capable of being liberated. In photoelectrochemical cells a p-type semiconductor is typically used as a photocathode the semiconductor responding to light by producing photo-excited electrons. Although various photoelectrodes have been developed, results to date have not been promising. It appears that the H.sub.2 evolution is limited by the rate of reaction, not the energy needed to drive it. Thus, there is a need for redox mediating agents which can accelerate this rate of reaction.
In running the hydrogen reaction in the reverse direction, for example, in fuel cells to generate electricity, activation of hydrogen can again be a problem. In a fuel cell, H.sub.2 is decomposed into 2H.sup.+ and 2e.sup.-, in which the electrons are then available for electricity. There also exists a need to improve this rate of reaction.